Talking to the Windows build system: config.w32

An extension's config.w32 file is similar in usage to
the config.m4 file, with two critical differences:
first, it is used for Windows builds, and second, it is written in
JavaScript. This section makes no attempt to cover JavaScript syntax. For
the moment, this section is incomplete in lieu of a Win32 testbed, and an
experimental-only port of the example config.m4 is the
only example provided.